The present Invention relates, in general, to millimeter and submillimeter wave amplifiers and, more particularly, to a novel traveling-wave amplifier which includes an integral active input/output circulator.
Information carrying systems such as radar and communications devices require an amplifier mechanism with substantial instantaneous bandwidth rather than simply an oscillator mechanism. In order to provide wide-band high power operation in traveling-wave amplifiers, the use of a tapered interaction waveguide in conjunction with a specially profiled magnetic field has been proposed in Application Ser. No. 06/389,133, filed June 16, 1982, entitled "Wide-Band Gyrotron Traveling-Wave Amplifier" by Y. Y. LAU, L. R. BARNETT, K. R. CHU, and V. L. GRANATSTEIN. The gyrotron traveling-wave amplifier disclosed therein comprises a tapered waveguide wherein the cross-section thereof gradually increases from a small first end to a larger second end for propagating electromagnetic energy therein, a magnetron injection gun for generating a beam of relativistic electrons with helical electron motion from the small first end of the tapered waveguide to propagate toward the larger second end in the axial direction therein, a magnetic circuit for generating a tapered magnetic field within the waveguide in a direction approximately parallel to the axis of the waveguide, and an input coupler for launching an input electromagnetic wave so that it co-propagates with the electron beam in the waveguide where the input wave is amplified.
The above-mentioned waveguide is tapered such that its cutoff frequency varies over a predetermined bandwidth. This device then utilizes a reverse RF injection scheme wherein the electromagnetic wave to be amplified is applied at the large end of the tapered waveguide so that it propagates in the waveguide toward the small end until it is reflected at the point in the waveguide taper where the individual frequency of the wave approximately matches the cutoff frequency of the waveguide. The reflected wave then co-propagates with and is amplified by the electron beam.
The reflection-type gyrotron traveling-wave amplifier disclosed in the above-referenced Application employed an external circulator device coupled to the single port formed by the large end of the tapered waveguide to separate the input and output of the amplifier. The use of an external circulator is disadvantageous because presently available wideband high power circulators for use at millimeter wavelengths do not exhibit sufficiently low standing wave ratios (VSWR) so as to allow stable high gain amplification.
An alternative input/output coupling technique is disclosed in Application Ser. No. 06/389,132, filed June 12, 1982, entitled "Wide-Band Distributed rf Coupler" by, L. R. BARNETT, Y. Y. LAU, K. R. CHU and V. L. GRANATSTEIN. The gyrotron traveling-wave amplifier disclosed therein comprises a distributed input coupler involving multiple-cavity coupling between an input waveguide and a tapered interaction waveguide. The coupler includes a plurality of channel filters distributed along the length of the tapered waveguide, with each channel filter comprising several coupled cavities in tandem for suppressing spurious modes.
The distributed coupler provides wide bandwidth coupling and thus complements the inherent broad-band characteristics of the gyrotron traveling-wave amplifier. The distributed coupler, however, is mechanically complex and thus is difficult to construct.
The present Invention provides a novel solution to the problem of coupling energy into and out of a traveling-wave amplifier.